The primary purpose of this study is to evaluate the safety of CP-195,543 in subjects with psoriasis vulgaris. CP-195,543 is a leukotriene b4 (LTB4)-inhibitor with demonstrated anti-inflammatory activity. Over one hundred subjects fulfilling eligibility criteria who have documented psoriasis will be randomized to receive six weeks of therapy with 50 or 25 mg CP-195,543 or placebo four times daily. A subset of the subjects on this study will also undergo a subcutaneous challenge with LTB4 on areas of skin not affected by psoriasis. At one week then biweekly intervals, all subjects will be evaluated for safety and toleration of therapy. The effect of CP-195,543 at these two doses on laboratory and clincal safety studies and on clinical manifestations and histopathology of psoriasis will be measured. Those subjects undergoing the LTB4 challenge will have cutaneous responses evaluated prior to and at the end of therapy. A secondary objective is to assess the anti-inflammatory activity of CP-195,543 to a direct cutaneous challenge with LTB4.